Recessed furnace lance purge gas system

Abstract

A system and method for protecting a nozzle that injects a high velocity main gas into a furnace or combustion zone. The nozzle is located in a passage recessed from the exit port opening of the passage which communicates with the combustion zone. Purge gas is passed into the passage upstream of the nozzle. The volume of the purge gas exceeds 50% of the total volume of the main gas and purge gas and the purge gas has a velocity of at least 100 ft/sec thereby preventing furnace gases from entering the passage and attacking the nozzle.

I claim: 
     
       1. A unit for injecting a main gas into a furnace combustion zone through a passage in a wall of the furnace having an exit port opining with a diameter D communicating with the combustion zone, the unit comprising: a nozzle in said passage positioned at a recess distance L from the exit port opening for injection of high velocity main gas into the passage toward the exit port opening; and   means for providing purge gas, said purge gas having a composition substantially the same as that of the high velocity main gas, into the passage at a point further recessed from the exit port opening than is the nozzle for flow of purge gas around and past the nozzle to prevent furnace gases from entering said passage and attacking the nozzle wherein the purge gas volume is greater than 50% of the total of the main gas and purge gas and the purge gas has a velocity greater than 100 ft/sec.   
     
     
       2. The unit as in claim 1 wherein L/D is within the range of from 0.5 to 3.0. 
     
     
       3. The unit as in claim 1 further comprising a conduit for supplying the main gas to said nozzle and wherein the means for providing purge gas comprises a conduit coaxial with the main gas conduit and having an exit terminating upstream of the nozzle. 
     
     
       4. A method for injecting gas into a furnace zone comprising: (A) passing main gas at a high velocity into a passage having an exit port opening which communicates with the furnace zone, said main gas being passed into the passage through a nozzle positioned at a recess from the opening;   (B) passing purge gas, said purge gas having a composition substantially the same as that of the main gas, into the passage at a point further recessed from the opening than where the main gas is passed into the passage, at a velocity exceeding 100 feet per second and at a flowrate such that the volume of purge gas passed into the passage is greater than 50 percent of the total volume of the main gas and purge gas passed into the passage; and   (C) passing the main gas and the purge gas from the passage through the exit port opening into the furnace zone.   
     
     
       5. The method of claim 4 wherein the main gas comprises oxidant. 
     
     
       6. The method of claim 4 wherein the main gas and the purge gas are passed through the exit port opening into the furnace zone at a flowrate which does not cause cooling of the exit port. 
     
     
       7. The method of claim 4 wherein the main gas and the purge gas are passed through the exit port opening into the furnace zone at a flowrate which does not cause accumulation of condensible material at the exit port. 
     
     
       8. A method for combusting fuel and oxidant in a combustion zone comprising: (A) passing main oxidant at a high velocity into a first passage having an exit port opening which communicates with the combustion zone, said main oxidant being passed into the first passage through a nozzle positioned at a recess from said first passage opening, and passing purge oxidant into the first passage at a point further recessed from said first passage opening than where the main oxidant is passed into the first passage, at a velocity exceeding 100 feet per second and at a flowrate such that the volume of purge oxidant passed into the first passage is greater than 50 percent of the total volume of the main oxidant and purge oxidant passed into the first passage;   (B) passing main fuel at a high velocity into a second passage having an exit port opening which communicates with the combustion zone, said main fuel passed into the second passage through a nozzle positioned at a recess from the opening and passing purge fuel into the second passage at a point further recessed from said second passage opening than where the main fuel is passed into the second passage, at a velocity exceeding 100 feet per second and at a flowrate such that the volume of purge fuel passed into the second passage is greater than 50 percent of the total volume of the main fuel and purge fuel passed into the second passage;   (C) passing the main oxidant and the purge oxidant from the first passage through the first passage exit port opening into the combustion zone, and passing the main fuel and the purge fuel from the second passage through the second passage exit port opening into the combustion zone; and   (D) mixing the main oxidant and the purge oxidant with the main fuel and the purge fuel in the combustion zone and combusting them therein.   
     
     
       9. A method for injecting gas into a furnace zone comprising: (A) passing main gas comprising fuel at a high velocity into a passage having an exit port opening which communicates with the furnace zone, said main gas being passed into the passage through a nozzle positioned at a recess from the opening;   (B) passing purge gas, said purge gas having a composition substantially the same as that of the main gas, into the passage at a point further recessed from the opening than where the main gas is passed into the passage, at a velocity exceeding 100 feet per second and at a flowrate such that the volume of purge gas passed into the passage is greater than 50 percent of the total volume of the main gas and purge gas passed into the passage; and   (C) passing the main gas and the purge gas from the passage through the exit port opening into the furnace zone.